Final answer:
An IPv6 datagram is dropped when encountering an IPv4-only router due to the challenge of protocol compatibility.
Step-by-step explanation:
When encountering an IPv4-only router, an IPv6 datagram is dropped. This statement reflects the challenges of protocol compatibility. IPv4 and IPv6 are two different network protocols and they are not fully compatible with each other. While IPv4 uses 32-bit addresses, IPv6 uses 128-bit addresses. Therefore, when an IPv6 datagram is sent to an IPv4-only router, it does not understand the new protocol and drops the datagram.